FILE - In this Dec. 23, 2015, file photo, 10-year-old Elison nurses his 2-month-old brother Jose Wesley, who was born with microcephaly, at their house in Poco Fundo, Pernambuco state, Brazil. The U.S. Centers for Disease Control and Prevention said Wednesday, Jan. 13, 2016, that it has found the strongest evidence so far of a possible link between a mosquito-borne virus and a surge of birth defects in Brazil. (AP Photo/Felipe Dana, File)

RIO DE JANEIRO (AP) — Researchers have found the strongest evidence so far of a possible link between a mosquito-borne virus and a surge of birth defects in Brazil, the U.S. Centers for Disease Control and Prevention said Wednesday.

The health agency said evidence of the dengue-like Zika virus was found in the placentas from two women who miscarried and the brains of two newborns who died. Those who were born had small heads, a rare condition known as microcephaly.

“The evidence is becoming very, very strong of the link between the two,” said Dr. Lyle Petersen, director of mosquito-borne diseases at the CDC.

Finding the virus present in brain tissue is “very significant,” he said.

Petersen warned that the link is not yet definite and said that a team of CDC investigators is traveling to Brazil in a few weeks to conduct more studies and learn what risks face pregnant women. “It’s possible that there may be some other co-factors involved.”

Zika is spread by the same Aedes mosquito that can carry dengue and chikungunya. There are no known cases of people contracting the virus in the U.S. mainland, though it has been seen in retrning travelers. Puerto Rico reported its first case of Zika two weeks ago, and 13 countries in Latin America have also seen infections.

The virus is related to dengue but until recently was thought to have only mild symptoms. It was first detected in humans in Uganda decades ago, but there had never been reports of links between the virus and brain malformations until recent months.

However, Brazil is experiencing the largest known outbreak of Zika, so it’s possible that a rare birth defect simply didn’t appear in previous smaller outbreaks, Petersen said.

“This is quite unique for this type of virus,” Petersen said. Mosquito-borne viruses generally don’t lead to neurological problems.

“I don’t think anybody has any idea how Zika is crossing the placenta into these fetuses, or why Zika is doing it and other closely related viruses like dengue don’t,” said Scott Weaver, director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston, Texas.

Dr. Ernesto Marques, an infectious diseases researcher at the University of Pittsburgh who is also studying Zika and the birth defects outbreak in Brazil, said he is finding so far that a very small percentage of pregnant women who reported Zika symptoms gave birth to babies with microcephaly.

He said a problem facing epidemiologists is the lack of labs that are able to test for Zika. Marques said estimates that the country had between 440,000 and 1.3 million cases of the virus in 2015 are not reliable.

“It needs to be better diagnosed. We don’t know many, and we need a better response,” he said.

Brazilian health authorities have said there’s no question Zika is behind the birth defects. The Health Ministry said late Tuesday that 3,530 babies have been born with microcephaly in the country since October. The number was less than 150 in 2014.

Now the government finds itself with a growing number of children with mental retardation concentrated in one of the poorest regions of the country in the northeast.

On Wednesday, Alberto Beltrame, national secretary of health care, said the government would invest $163 million to provide care and physical and speech therapy to those babies through the first three years.

Beltrame said the government wants to train more than 7,500 physical therapists, doctors and psychologists in techniques to help develop motor and language skills in infants and toddlers with microcephaly.

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